1. Field of the Invention
The present invention relates generally to flow control valves, and more particularly to an improved proportional flow control valve.
2. Description of the Related Art
Continuous sheet casting processes are well known in the art for producing elongated tapes or sheets of material of a desired thickness. For example, it is well known to cast ceramic tape by depositing a liquid slurry, containing the ceramic material dissolved in a solvent, onto a moving belt, passing the belt under a xe2x80x9cdoctorxe2x80x9d blade to limit the thickness of the slurry, and curing the tape by driving off the solvent in a heating step to leave an elongated tape of solid ceramic material. Such a ceramic tape might be used, for example, in the manufacture of interconnect substrates for micro-electronics.
In known sheet casting processes of this general type, the most common method of controlling the thickness of the tape is to control the height of the aforementioned xe2x80x9cdoctorxe2x80x9d blade. A feedback control system has been used wherein the height of the tape is determined on the downstream side of the xe2x80x9cdoctor blade,xe2x80x9d i.e., after the tape has passed beyond the xe2x80x9cdoctorxe2x80x9d blade. In some systems, the tape height is determined just beyond the xe2x80x9cdoctorxe2x80x9d blade when the tape is still xe2x80x9cwetxe2x80x9d; in other systems, the tape height is determined further downstream after the tape has cured. In either case, information relating to the tape height is used to control the height of the xe2x80x9cdoctorxe2x80x9d blade above the moving belt in order to maintain the tape at a desired thickness. However, raising and lowering the height of the xe2x80x9cdoctorxe2x80x9d blade only goes so far in producing a cast tape of uniform thickness.
Known casting systems often include valves for regulating the deposition of the slurry onto the moving belt of the casting system. The slurry is supplied from a pressurized source, or is pumped from an un-pressurized source, through a flexible tube. As the slurry can be abrasive and damaging to common types of control valves, the flow of slurry is typically controlled by a xe2x80x9cpinch-offxe2x80x9d control valve that extends about the flexible slurry supply tube. This pinch-off valve is typically either open (i.e., out of contact with the supply tube) or closed (i.e., it pinches off the supply tube). The slurry passing through the control valve is discharged into a reservoir located above the moving belt. In turn, the reservoir releases slurry onto the moving belt upstream from the xe2x80x9cdoctorxe2x80x9d blade. It is known to include a sensor for sensing the slurry level in the reservoir to open or close the control valve in order to selectively discharge additional slurry into the reservoir. However, this process of regulating the level of slurry in the reservoir tends to be a discontinuous, start-and-stop process, which does not contribute to the production of a tape with uniform thickness. In addition, the application of excessive force to the supply tube during pinch-off (excessive crushing force) can prevent the supply tube from fully-opening when full-flow is later needed. Another problem that arises is that, depending on the type of tape being cast and the nature of the slurry, the diameter of the slurry supply tube that fills the slurry reservoir can vary, thereby changing the requirements of the pinch-off valve.
Moreover, solvent-based slurries pose a risk of explosion, as the solvent fumes are often volatile. Known casting systems that sense the height of the tape downstream from the xe2x80x9cdoctorxe2x80x9d blade use electronic sensors that are positioned in close proximity to the tape. However, such electronic sensors are subject to the creation of electrical sparks that, under some circumstances, could cause a dangerous and damaging explosion.
Accordingly, it is an object of the present invention to provide a control valve for dispensing a slurry or other fluid wherein the discharge of such fluid can easily be regulated in a linear, or xe2x80x9cproportional control,xe2x80x9d fashion.
It is another object of the present invention to provide such a control valve that is relatively inexpensive and simple to construct.
A further object of the present invention is to provide such a control valve wherein the supply of slurry to the moving belt can be better regulated in a continuous manner as opposed to a start-and-stop manner.
A yet further object of the present invention is to provide such a control valve which is easy to adjust, and which can be quickly converted for use with supply tubes of varying diameter.
Another object of the present invention is to provide such a control valve which prevents excessive crushing forces from being applied to the slurry supply tube, even when all flow must be pinched off.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.
Briefly described, and in accordance with one aspect thereof, the present invention relates to a proportional flow control valve for controlling the flow of a fluid through a flexible tube, and including a pair of jaws for engaging opposing sides of the flexible tube, as well as an actuator coupled to at least one of such jaws for varying the separation distance between the jaws. The actuator varies the separation distance between an initial separation distance and a shut-off separation distance. When the jaws are separated by the initial separation distance, the flexible tube is at least partially deformed, but avoids significant restriction of the flow of fluid therethrough. On the other hand, the shut-off separation distance is just restrictive enough to essentially block the flow of fluid through the flexible tube. A control system is used to control the actuator in order to select the separation distance between the jaws at any given time.
Preferably, the aforementioned proportional flow control valve includes a manual adjustment coupled to at least one of the jaws for adjusting the initial separation distance, i.e., for adjusting the amount by which the flexible tube is pre-crushed by the jaws without significantly restricting flow. The aforementioned actuator can advantageously be implemented by a ram operated by a pressurized fluid, and coupled to one of the jaws for selectively urging the jaws together. In this case, the actuator control system regulates the pressure applied to the pressurized fluid that operates the ram, e.g., a pneumatic pressure regulator that applies pneumatic pressure to the actuator. A particularly simple and straightforward control system regulates the fluid pressure to have a magnitude substantially inversely proportional to the flow of fluid through the flexible tube. In other words, if the pneumatic pressure applied to the actuator doubles, the flow of slurry or other fluid through the flexible tube is reduced by half.
If the actuator control system decreases fluid pressure, then it is desired that the jaws open up to increase fluid flow through the flexible tube. Accordingly, the proportional flow control valve preferably includes a bias member coupled to the actuator to apply a force to the actuator tending to increase the separation distance between the jaws. It has been found that a compressible spring washer, or preferably, a series of successive compressible spring washers serve this purpose well by generating a biasing force that varies substantially linearly with the amount of displacement of the actuator. Such spring washers become compressed when the actuator decreases the separation distance between said first and second jaws, and then force the jaws apart again when the pressure of the pressurized fluid operating on the actuator is later released. This bias force can easily be adjusted by providing a selector to select the number of spring washers that are compressed when the actuator decreases the separation distance between the jaws. Ideally, the selector is operated to select a number of such spring washers such that the biasing force developed by the selected spring washers under compression is commensurate with the predetermined shut-off force that is applied to the actuator ram when the separation distance between the jaws is decreased to the shut-off separation distance.
In a preferred embodiment of such proportional flow control valve, the actuator ram is secured to, and moves, one jaw relative to the other jaw; a first adjustable stop is secured to the actuator ram for limiting the travel of the movable jaw relative to the fixed jaw, thereby preventing the flexible tube from being overly crushed. A similar adjustable stop may also be provided to limit the travel of the actuator ram in the opposite direction, thereby limiting the degree to which the bias member can force the movable jaw away from the fixed jaw.
Applicant has found that the above-described flow control valve operates in a substantially linear, proportional manner; thus, when the fixed and movable jaws are midway between the initial separation distance and the shut-off separation distance, the flow through the flexible tube is approximately one-half that when the jaws are separated by the initial separation distance.